1. Field of the Invention
The present invention relates to a piezoelectric resonator adapted to generate a harmonic wave in a thickness extensional vibration mode. More particularly, the present invention relates to the above-described piezoelectric resonator including a plurality of excitation electrodes superposed on one another in a thickness direction of the resonator with piezoelectric layers interposed between the excitation electrodes.
2. Description of the Related Art
A laminated piezoelectric resonator utilizing a harmonic wave in a thickness extensional vibration mode is known. The piezoelectric resonator of this type is disclosed in Japanese Patent Laid-Open Publication No. 176110/1989, for example, and is shown in the exploded perspective view of FIG. 10. This piezoelectric resonator has an excitation electrode 52 disposed at a central portion on an upper surface of a piezoelectric layer 51, an excitation electrode 54 disposed at a central portion on an upper surface of a piezoelectric layer 53, and an excitation electrode 55 disposed at a central portion on a lower surface of the piezoelectric layer 53. In FIG. 10, the excitation electrode 55 is shown by being projected on a plane at the bottom.
Thin lead-out electrodes 52a, 54a and 55a are arranged to extend continuously from the excitation electrodes 52, 54, and 55, respectively. Each of the lead-out electrode 52a, 54a and 55a extends to an edge of the piezoelectric layer 51 or 53.
As shown in FIG. 11, in this piezoelectric resonator, the portions of excitation electrodes 52, 54 and 55 superposed on one another and the portions of piezoelectric layers 51 and 53 interposed between the electrodes constitute a vibrating section, in which a thickness extensional vibration is excited at a harmonic frequency when an alternating current is applied between the excitation electrode 54 and the excitation electrodes 52 and 55.
However, the above-described piezoelectric resonator experiences a problem in that vibration in an oblique symmetrical mode (A mode) occurs when the resonator is driven. The A mode vibration appears as a ripple in the resonance characteristic.
To manufacture the above-described piezoelectric resonator, a mother piezoelectric body 56 corresponding to a planar array of a plurality of laminate pieces formed of piezoelectric layers 51 and 53 as shown in FIG. 12 is provided. Unit laminates each corresponding to one piezoelectric resonator are obtained by cutting the mother piezoelectric body 56 along broken lines A to C in FIG. 12 parallel to the thickness direction. In FIG. 12, the electrode films formed in the piezoelectric body 56 are indicated at 58 while excitation electrodes 52 and lead-out electrodes 52a formed on the upper surface of the mother piezoelectric body 56 are not shown.
Electrode films 58 are provided to form the internal excitation electrodes 54 and lead-out electrodes 54a shown in FIG. 10. Edges of lead-out electrodes 54a can be reliably exposed at side surfaces of piezoelectric resonators 57 by cutting along broken lines B.
That is, after preparing the mother piezoelectric body 56 shown in FIG. 12, end surfaces of lead-out electrodes 54a are exposed by cutting internal electrode films 58 along broken lines B. By this cutting step, certain piezoelectric body portions shown between broken lines B and C in FIG. 12 must be removed, which requires unnecessary consumption of the piezoelectric material and the electrode material. This limits attempts to reduce manufacturing cost.
Thus, the above-described conventional piezoelectric resonator adapted to vibrate in the thickness extensional vibration mode has the disadvantage of having a ripple due to an oblique symmetrical mode and having an increased manufacturing cost due to unnecessary consumption of the piezoelectric and electrode materials in a mass production process.